1. Field of the Invention
The invention relates to apparatus for automatically spotting substrates such as thin layer chromatographic sheets. Known quantities of mixtures of chemical compounds which are to be analyzed by a comparison of their relative rates of flow through the coating under the influence of a migrating solvent are spotted on chromatographic sheets. Such sheets generally comprise a sorbent such as a silica gel, a binder such as sodium polyacrylate, and a thicker coated on a support. Because the spot enlarges upon migration, the spot initially must be very small, i.e., no larger than about 5 mm in diameter. As volumes required for such analysis generally exceed those which will form such a small spot, repeated spotting and drying of the same area of the substrate is usually necessary.
2. State of the Prior Art
Spotting of thin layer chromatographic sheets, hereinafter "TLC" sheets, by successive contacts to form a spot of proper concentration has long been done by hand. If large amounts of solution are involved, such manual labor is at best very time consuming.
Many automated approaches have been suggested to obtain the required spotting. In an attempt to mechanically duplicate the manual process of repeatedly touching a capillary tube perpendicularly to a sheet, containers have been mounted in holding bars which are brought to bear against the sheet to be spotted. Such containers have included cups, syringes, and capillary tubes, as are shown for example in U.S. Pat. Nos. 3,189,413; 3,758,275; 3,766,884; and 3,833,341 and in German DT 2,208,362. Not only have such devices been quite complex in construction, as shown in U.S. Pat. No. 3,189,413, they also have not specifically provided for the thin-skinned nature of the surface of the sheet or substrate to be spotted. That is, such sheets have coatings which are soft when wet and are easily damaged, i.e., flaked or other-wise removed, by mechanical forces indiscriminately applied by automated applicators. Such damage is particularly likely when the spotting devices are provided with sharp edges against which the sheet is moved, as in stripe lay-down, e.g., in a manner such as is disclosed in U.S. Pat. No. 3,358,496. It has been suggested that springs be used to ease the spotters into position, as is disclosed in U.S. Pat. No. 3,780,700. However, once engaged, such devices still bring to bear a large, fixed pressure incapable of change as might be necessary to accommodate a variance in sheet thickness. Such variance can be caused, for example, when sheets having different types of supports are used.
If the apparatus parts are carefully machined, when the capillary tubes are moved into contact with the substrate, they cannot be moved or pressed any farther and excessive pressure is avoided. Such close tolerances however are expensive to provide, and because of wear, are difficult to maintain.
Another approach has been to provide the test solution as a drop which falls onto the sheet rather than as a solution touched to the sheet. However, such devices, as are described, for example, in U.S. Pat. No. 3,843,053, are susceptible to wide variations in spot size, and are not suitable to the production of very small spots. Yet another approach has been to provide the capillary tubes or syringe needles with bending moment flexibility so that they can be bent to touch down into a sheet, as is shown in U.S. Pat. No. 3,738,493 and in Canadian Pat. No. 933,746. However, such devices do not place the confining walls of the capillary orifice at right angles to the sheet, as is required for minimum dispersion and maximum diffusion-to-flow ratio, or they require specially bias-cut tubes which do not provide such right angle contact.
Still another approach has been to wick the solution onto the sheet as shown, for example, in U.S. Pat. No. 3,568,634. However, residual solution may stay in the wick thereby giving rise to a false reading.
Apparatus has been constructed which provides perpendicular tube-to-sheet contact while incidentally reducing the applicator to sheet pressure. For example, in U.S. Pat. No. 3,807,959, there is disclosed apparatus comprising a plurality of sample-carrying capillary tubes clamped in a bar which in turn slides under the influence of gravity between two notched supports towards the TLC plate to be spotted. Because the bar is free to move relative to the plate being spotted, the maximum pressure applied at each capillary tube results from the weight of the bar, the tubes, and their contents. However, the massive size of the bar relative to the tubes renders this pressure of significant magnitude, and this must be reduced by tilting the support for the plate or sheet so that the sheet is nearly vertical and the bar is only slightly tilted out of the horizontal. Furthermore, such apparatus suffers the disadvantage of requiring perfect alignment of the ends of the capillary tubes in a plane parallel to the plane of the sheet, prior to clamping of the bar, as otherwise any tube which projects out of the plane will prevent the other tubes from properly contacting the sheet.
Chromatographic applicators not suitable for spot formation include hand-held devices such as shown in U.S. Pat. No. 3,495,446.
Patents directed to the background of devices for forming spots or coatings in general include U.S. Pat. Nos. 3,839,183; 3,428,547; 3,404,025; 3,317,418; 3,260,413; 2,868,020 and 2,832,140.